In the manufacture of certain products, notably printed circuit boards, it is important that the wall angles of the edges of laser-ablated film portions be correct. For example, if the wall angle of a via is too steep, evaporated metal will not adhere to the wall in amounts sufficient to prevent discontinuities in the electrical circuit. On the other hand, if a via has a wall angle that is too shallow, the via will occupy more space in the circuit board than is necessary.
Prior art workers have attempted to control wall angle by controlling the fluence level in the beam from the excimer laser. Typically, in the ablation process relative to an organic polymer dielectric, the threshold fluence required for ablation is approximately 70 mJ/cm.sup.2 per pulse to 100 mJ/cm.sup.2 per pulse. The required threshold fluence depends on the particular polymer (such as a polyimide) being ablated (reference is made to "Excimer Laser Etching of Polyimide", by J. H. Brannon, J. R. Lankard, A. I. Baise, F. Burns, and J. Kaufman, Journal of Applied Physics, Volume 58, No. 5, 1 Sep. 1985). When the fluence is raised from 1.1 times threshold to 2.0 times threshold, the wall angle increases from about 65.degree. to about 80.degree. from the plane of the board. However, this entire range of angles is undesirably high for the walls of vias in printed circuit boards, in that wall angles in the range of about 45.degree. to about 50.degree. from the plane of the board are generally regarded as optimum.
Prior art workers have also attempted to control wall angle by defocusing the projection lens, but this approach has achieved only limited success. It has been found that in order to have a wall angle of 50.degree. or less, from the plane of the board, the projection lens must be moved far out of focus for a relatively large diameter via, but must be moved only a minor amount out of focus for a small diameter via. Thus, if openings for both large-size vias and small-size vias are required in one mask, a distinct problem is presented. In fact, if small and large openings are provided in a single mask, and the defocusing approach is employed, relatively satisfactory wall angles are provided relative to the large features, but the small or fine features are lost. Resolution capability is thus very severely affected.